1. Field of the Invention
The invention relates to a brake systems for motor vehicles. More specifically, the invention relates to a brakes system with an actuating device, which is controlled by a brake pedal through electric lines. The actuating device is in the form of an electromechanical wheel brake actuator mounted on the brake caliper of each wheel.
2. Description of the Related Art
Increasing demands now made of motor vehicles in terms of a modern brake system--such as ABS, driving stability controls, drive-slip or traction controls--make wheel-selective brake interventions necessary. This has been accomplished in the prior art with conventional brake systems that have been supplemented with hydraulic pumps and magnet valves (German Patent DE-C 29 54 162). Then, however, vibration problems in the hydraulic lines and difficult triggering of the pressure modulation units, in other words magnet valves, arise. Because of the properties of the magnet valves, which are highly nonlinear two-position members, the control quality with respect to the brake pressure is also limited. Such brake systems also require considerable effort and expense to install in the motor vehicle: brake lines have to be laid and connected, the brake system has to be filled with brake fluid and bled, and the tightness of the system has to be checked. During operation as well, considerable maintenance expense arises, particularly for regularly replenishing the brake fluid and disposing of it in an environmentally appropriate way.
Currently, all known manufacturers still equip their vehicles with conventional brake systems. Wheel-selective brake interventions are effected by means of hydraulic pumps and solenoid valves, at the cost of the above-addressed disadvantages. For gentle brake pressure buildup--for instance in cruise control and tailgating control systems--some manufacturers use electronically regulated negative pressure brake boosters. To suppress the vibration problems and the attendant noise production, proportional valves and pressure reservoirs can be used, but the proportional valves in particular increase the price for the brake system. Nor does this eliminate the disadvantages associated with the hydraulic fluid.
One known brake actuating system for a motor vehicle (German patent disclosure DE 42 29 042 A1) has one electromechanical wheel brake actuator mounted on the brake caliper of each wheel and containing a spindle, driven in the direction of the axle, and an electric motor. The rotor of the electric motor is embodied as a spindle nut of a spindle gear that converts the rotary motion of the rotor into a linear motion of the spindle. The axial force of the spindle is multiplied by a hydraulic step-up gear and transmitted to a piston of a wheel brake cylinder. Readjustment, which is necessary to compensate for wear to the brake linings, is accomplished in this brake actuating system by means of brake fluid flowing from a supply container into the wheel brake cylinder.
Upon electromechanical actuation of the brakes, since there is no hydraulic fluid available, some other readjustment option must be provided for. In a mechanical configuration for automatic compensation of brake lining wear for a mechanically actuated parking brake (European patent disclosure EP 0 403 635 B1), the readjustment is accomplished with the aid of the hydraulic forces that are generated, upon actuation of a disk brake, via a brake pedal, a braking unit, and a master cylinder. The readjustment is effected when the brake is loaded, which requires an intermediate piston.